Organometallic compounds for use in electroluminescent devices

ABSTRACT

An organic light emitting device having an anode, a cathode and an organic layer between the anode and the cathode is provided. The organic layer comprises a carbene-metal complex having the structure:

This application is a continuation-in-part of U.S. application Ser. No.10/880,384, filed Jun. 28, 2004, now U.S. Pat. No. 7,393,599, entitledLuminescent Compounds with Carbene Ligands, which is acontinuation-in-part of U.S. application Ser. No. 10/849,301, filed May18, 2004, and both of which are incorporated by reference in itsentirety.

The claimed invention was made by, on behalf of, and/or in connectionwith one or more of the following paflies to a jointuniversity-corporation research agreement: Princeton University, TheUniversity of Southern California, and the Universal DisplayCorporation. The agreement was m effect on and before the date theclaimed invention was made, and the claimed invention was made as aresult of activities undertaken within the scope of the agreement.

FIELD OF THE INVENTION

The present invention relates to organic light emitting devices (OLEDs),and more specifically to phosphorescent organic materials used in suchdevices. More specifically, the present invention relates tocarbene-metal complexes incorporated into OLEDs.

BACKGROUND

Opto-electronic devices that make use of organic materials are becomingincreasingly desirable for a number of reasons. Many of the materialsused to make such devices are relatively inexpensive, so organicopto-electronic devices have the potential for cost advantages overinorganic devices. In addition, the inherent properties of organicmaterials, such as their flexibility, may make them well suited forparticular applications such as fabrication on a flexible substrate.Examples of organic opto-electronic devices include organic lightemitting devices (OLEDs), organic phototransistors, organic photovoltaiccells, and organic photodetectors. For OLEDs, the organic materials mayhave performance advantages over conventional materials. For example,the wavelength at which an organic emissive layer emits light maygenerally be readily tuned with appropriate dopants.

As used herein, the term “organic” includes polymeric materials as wellas small molecule organic materials that may be used to fabricateorganic opto-electronic devices. “Small molecule” refers to any organicmaterial that is not a polymer, and “small molecules” may actually bequite large. Small molecules may include repeat units in somecircumstances. For example, using a long chain alkyl group as asubstituent does not remove a molecule from the “small molecule” class.Small molecules may also be incorporated into polymers, for example as apendent group on a polymer backbone or as a part of the backbone. Smallmolecules may also serve as the core moiety of a dendrimer, whichconsists of a series of chemical shells built on the core moiety. Thecore moiety of a dendrimer may be a fluorescent or phosphorescent smallmolecule emitter. A dendrimer may be a “small molecule,” and it isbelieved that all dendrimers currently used in the field of OLEDs aresmall molecules. In general, a small molecule has a well-definedchemical formula with a single molecular weight, whereas a polymer has achemical formula and a molecular weight that may vary from molecule tomolecule. As used herein, “organic” includes metal complexes ofhydrocarbyl and heteroatom-substituted hydrocarbyl ligands.

OLEDs make use of thin organic films that emit light when voltage isapplied across the device. OLEDs are becoming an increasinglyinteresting technology for use in applications such as flat paneldisplays, illumination, and backlighting. Several OLED materials andconfigurations are described in U.S. Pat. Nos. 5,844,363, 6,303,238, and5,707,745, which are incorporated herein by reference in their entirety.

OLED devices are generally (but not always) intended to emit lightthrough at least one of the electrodes, and one or more transparentelectrodes may be useful in organic opto-electronic devices. Forexample, a transparent electrode material, such as indium tin oxide(ITO), may be used as the bottom electrode. A transparent top electrode,such as disclosed in U.S. Pat. Nos. 5,703,436 and 5,707,745, which areincorporated by reference in their entireties, may also be used. For adevice intended to emit light only through the bottom electrode, the topelectrode does not need to be transparent, and may be comprised of athick and reflective metal layer having a high electrical conductivity.Similarly, for a device intended to emit light only through the topelectrode, the bottom electrode may be opaque and/or reflective. Wherean electrode does not need to be transparent, using a thicker layer mayprovide better conductivity, and using a reflective electrode mayincrease the amount of light emitted through the other electrode, byreflecting light back towards the transparent electrode. Fullytransparent devices may also be fabricated, where both electrodes aretransparent. Side emitting OLEDs may also be fabricated, and one or bothelectrodes may be opaque or reflective in such devices.

As used herein, “top” means furthest away from the substrate, while“bottom” means closest to the substrate. For example, for a devicehaving two electrodes, the bottom electrode is the electrode closest tothe substrate, and is generally the first electrode fabricated. Thebottom electrode has two surfaces, a bottom surface closest to thesubstrate, and a top surface further away from the substrate. Where afirst layer is described as “disposed over” a second layer, the firstlayer is disposed further away from substrate. There may be other layersbetween the first and second layer, unless it is specified that thefirst layer is “in physical contact with” the second layer. For example,a cathode may be described as “disposed over” an anode, even thoughthere are various organic layers in between.

As used herein, “solution processible” means capable of being dissolved,dispersed, or transported in and/or deposited from a liquid medium,either in solution or suspension form.

As used herein, and as would be generally understood by one skilled inthe art, a first “Highest Occupied Molecular Orbital” (HOMO) or “LowestUnoccupied Molecular Orbital” (LUMO) energy level is “greater than” or“higher than” a second HOMO or LUMO energy level if the first energylevel is closer to the vacuum energy level. Since ionization potentials(IP) are measured as a negative energy relative to a vacuum level, ahigher HOMO energy level corresponds to an IP having a smaller absolutevalue (an IP that is less negative). Similarly, a higher LUMO energylevel corresponds to an electron affinity (EA) having a smaller absolutevalue (an EA that is less negative). On a conventional energy leveldiagram, with the vacuum level at the top, the LUMO energy level of amaterial is higher than the HOMO energy level of the same material. A“higher” HOMO or LUMO energy level appears closer to the top of such adiagram than a “lower” HOMO or LUMO energy level.

The carbene ligand has been well known in organometallic chemistry, andis used to generate a wide range of thermally stable catalyticmaterials. The carbene ligands have been employed both as active groups,directly engaged in the catalytic reactions, and serving a role ofstabilizing the metal in a particular oxidation state or coordinationgeometry. However, applications of carbene ligands are not well known inphotochemistry.

One issue with many existing organic electroluminescent compounds isthat they are not sufficiently stable for use in commercial devices. Anobject of the invention is to provide a class of organic emissivecompounds having improved stability.

In addition, existing compounds do not include compounds that are stableemitters for high energy spectra, such as a blue spectra. An object ofthe invention is to provide a class of organic emissive compounds thatcan emit light with various spectra, including high energy spectra suchas blue, in a stable manner.

SUMMARY OF THE INVENTION

An organic light emitting device is provided. The device has an anode, acathode and an organic layer disposed between the anode and the cathode.The organic layer comprises a compound further comprising one or morecarbene ligands coordinated to a metal center having the followingstructures:

in which M is a metal; R₁ is independently selected from hydrogen,alkyl, alkenyl, alkynyl, aralkyl, aryl, heteroaryl, substituted aryl,substituted heteroaryl, or a heterocyclic group; R₂, R₃, R₈, R₁₀, R₁₁,and R₁₂ are independently selected from hydrogen, alkyl, alkenyl,alkynyl, aralkyl, CN, CF₃, CO₂R′, C(O)R′C(O)NR′₂, NR′₂NO₂, OR′, SR′,SO₂, SOR′, SO₃R′, halo, aryl, heteroaryl, substituted aryl, substitutedheteroaryl, or a heterocyclic group; and additionally or alternatively,one or more of R₁ and R₂, and R₂ and R₃, and two R₈ groups, two R₁₀groups, and two R₁₂ groups on the same ring together form independentlya 5 or 6-member cyclic group, wherein said cyclic group is cycloalkyl,cycloheteroalkyl, aryl or heteroaryl; and wherein said cyclic group isoptionally substituted by one or more substituents J; each substituent Jis independently selected from the group consisting of R′, CN, CF₃,C(O)OR′, C(O)R′, C(O)NR′₂, NR′2, NO₂, OR′, SR′, SO₂, SOR′, or SO₃R′, andadditionally, or alternatively, two J groups on adjacent ring atoms forma fused 5- or 6-membered aromatic group; each R′ is independentlyselected from halo, H, alkyl, alkenyl, alkynyl, heteroalkyl, aralkyl,aryl and heteroaryl; (X—Y) is selected from a photoactive ligand or anancillary ligand; d is 0, 1, 2, 3, or 4; m is a value from 1 to themaximum number of ligands that may be attached to metal M; m+n is themaximum number of ligands that may be attached to metal M.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an organic light emitting device having separate electrontransport, hole transport, and emissive layers, as well as other layers.

FIG. 2 shows an inverted organic light emitting device that does nothave a separate electron transport layer.

FIG. 3 shows plots of current vs. voltage for deviceCuPc(100)/NPD(300)/UGH5: Dopant A (6%, 300)/BAlQ(400) and deviceCuPc(100)/NPD(300)/UGH5: Dopant B (6%, 300)/BAlQ(400).

FIG. 4 shows plots of quantum efficiency vs. current density for deviceCuPc(100)/NPD(300)/UGH5: Dopant A (6%, 300)/BAlQ(400) and deviceCuPc(100)/NPD(300)/UGH5: Dopant B (6%, 300)/BAlQ(400).

FIG. 5 shows plots of the electroluminescent spectra of deviceCuPc(100)/NPD(300)/UGH5: Dopant A (6%, 300)/BAlQ(400) and deviceCuPc(100)/NPD(300)/UGH5: Dopant B (6%, 300)/BAlQ(400).

FIG. 6 shows plots of current vs. voltage for deviceCuPc(100)/NPD(300)/mCBP: Dopant C (6%, 300)/BAlQ(400) and deviceCuPc(100)/NPD(300)/mCBP:Dopant D (6%, 300)/BAlQ(400).

FIG. 7 shows plots of quantum efficiency vs. current density for deviceCuPc(100)/NPD(300)/mCBP: Dopant C (6%, 300)/BAlQ(400) and deviceCuPc(100)/NPD(300)/mCBP: Dopant D (6%, 300)/BAlQ(400).

FIG. 8 shows plots of the electroluminescent spectra of deviceCuPc(100)/NPD(300)/mCBP: Dopant C (6%, 300)/BAlQ(400) and deviceCuPc(100)/NPD(300)/mCBP: Dopant D (6%, 300)/BAlQ(400).

FIG. 9 shows plots of current vs. voltage for deviceCuPc(100)/NPD(300)/mCBP: Dopant E (300, 6%)/BAlQ(400) and deviceCuPc(100)/NPD(300)/mCBP: Dopant F (300, 6%)/BAlQ(400).

FIG. 10 shows plots of quantum efficiency vs. current density for deviceCuPc(100)/NPD(300)/mCBP: Dopant E (300, 6%)/BAlQ(400) and deviceCuPc(100)/NPD(300)/mCBP: Dopant F (300, 6%)/BAlQ(400).

FIG. 11 shows plots of the electroluminescent spectra of deviceCuPc(100)/NPD(300)/mCBP: Dopant E (300, 6%)/BAlQ(400) and deviceCuPc(100)/NPD(300)/mCBP: Dopant F (300, 6%)/BAlQ(400).

FIG. 12 shows plots of current vs. voltage for device CuPc(100)/NPD(300)/UGH5: Dopant G (6%, 300)/BAlQ(400).

FIG. 13 shows plots of quantum efficiency vs. current density forCuPc(100)/NPD(300)/UGH5: Dopant G (6%, 300)/BAlQ(400).

FIG. 14 shows plots of the electroluminescent spectra ofCuPc(100)/NPD(300)/UGH5: Dopant G (6%, 300)/BAlQ(400).

FIG. 15 shows plots of current vs. voltage for deviceCuPc(100)/NPD(300)/mCP: Dopant H (6%, 300)/BAlQ(400), deviceCuPc(100)/NPD(300)/CBP:dopant132(6%, 300)/BAlQ(400), and deviceCuPc(100)/NPD(300)/mCBP:dopant130(6%, 300)/BAlQ(400).

FIG. 16 shows plots of quantum efficiency vs. current density for deviceCuPc(100)/NPD(300)/mCP: Dopant H (6%, 300)/BAlQ(400), deviceCuPc(100)/NPD(300)/CBP: Dopant I (6%, 300)/BAlQ(400), and deviceCuPc(100)/NPD(300)/mCBP: Dopant J (6%, 300)/BAlQ(400).

FIG. 17 shows plots of the electroluminescent spectra of deviceCuPc(100)/NPD(300)/mCP: Dopant H (6%, 300)/BAlQ(400), deviceCuPc(100)/NPD(300)/CBP: Dopant I (6%, 300)/BAlQ(400), and deviceCuPc(100)/NPD(300)/mCBP: Dopant J (6%, 300)/BAlQ(400).

FIG. 18 shows plots of current vs. voltage for deviceCuPc(100)/NPD(300)/mCBP: Dopant K (6%, 300)/BAlQ(400), deviceCuPc(100)/NPD(300)/mCBP: Dopant L (6%, 300)/BAlQ(400), and deviceCuPc(100)/NPD(300)/UGH5: Dopant M (6%, 300)/BAlQ(400).

FIG. 19 shows plots of quantum efficiency vs. current density for deviceCuPc(100)/NPD(300)/mCBP: Dopant K (6%, 300)/BAlQ(400), deviceCuPc(100)/NPD(300)/mCBP: Dopant L (6%, 300)/BAlQ(400), and deviceCuPc(100)/NPD(300)/UGH5: Dopant M (6%, 300)/BAlQ(400).

FIG. 20 shows plots of the electroluminescent spectra of deviceCuPc(100)/NPD(300)/mCBP: Dopant K (6%, 300)/BAlQ(400), deviceCuPc(100)/NPD(300)/mCBP: Dopant L (6%, 300)/BAlQ(400), and deviceCuPc(100)/NPD(300)/UGH5: Dopant M (6%, 300)/BAlQ(400).

FIG. 21 shows plots of current vs. voltage for deviceCuPc(100)/NPD(300)/UGH5: Dopant N (6%, 300)/BAlQ(400), deviceCuPc(100)/NPD(300)/UGH5: Dopant O (6%, 300)/BAlQ(400), and deviceCuPc(100)/NPD(300)/UGH5: Dopant P (6%, 300)/BAlQ(400).

FIG. 22 shows plots of quantum efficiency vs. current density for deviceCuPc(100)/NPD(300)/UGH5: Dopant N (6%, 300)/BAlQ(400), deviceCuPc(100)/NPD(300)/UGH5: Dopant O (6%, 300)/BAlQ(400), and deviceCuPc(100)/NPD(300)/UGH5: Dopant P (6%, 300)/BAlQ(400).

FIG. 23 shows plots of the electroluminescent spectra of deviceCuPc(100)/NPD(300)/UGH5: Dopant N (6%, 300)/BAlQ(400), deviceCuPc(100)/NPD(300)/UGH5: Dopant O (6%, 300)/BAlQ(400), and deviceCuPc(100)/NPD(300)/UGH5: Dopant P (6%, 300)/BAlQ(400).

FIG. 24 shows plots of the plot of operation lifetime of deviceCuPc(100)/NPD(300)/UGH5: Dopant D (6%, 300)/BAlQ(400).

DETAILED DESCRIPTION

Generally, an OLED comprises at least one organic layer disposed betweenand electrically connected to an anode and a cathode. When a current isapplied, the anode injects holes and the cathode injects electrons intothe organic layer(s). The injected holes and electrons each migratetoward the oppositely charged electrode. When an electron and holelocalize on the same molecule, an “exciton,” which is a localizedelectron-hole pair having an excited energy state, is formed. Light isemitted when the exciton relaxes via a photoemissive mechanism. In somecases, the exciton may be localized on an excimer or an exciplex.Non-radiative mechanisms, such as thermal relaxation, may also occur,but are generally considered undesirable.

The initial OLEDs used emissive molecules that emitted light from theirsinglet states (“fluorescence”) as disclosed, for example, in U.S. Pat.No. 4,769,292, which is incorporated by reference in its entirety.Fluorescent emission generally occurs in a time frame of less than 10nanoseconds.

More recently, OLEDs having emissive materials that emit light fromtriplet states (“phosphorescence”) have been demonstrated. Baldo et al.,“Highly Efficient Phosphorescent Emission from OrganicElectroluminescent Devices,” Nature, vol. 395, 151-154, 1998;(“Baldo-I”) and Baldo et al., “Very high-efficiency green organiclight-emitting devices based on electrophosphorescence,” Appl. Phys.Lett., vol. 75, No. 3, 4-6 (1999) (“Baldo-II”), which are incorporatedby reference in their entireties. Phosphorescence may be referred to asa “forbidden” transition because the transition requires a change inspin states, and quantum mechanics indicates that such a transition isnot favored. As a result, phosphorescence generally occurs in a timeframe exceeding at least 10 nanoseconds, and typically greater than 100nanoseconds. If the natural radiative lifetime of phosphorescence is toolong, triplets may decay by a non-radiative mechanism, such that nolight is emitted. Organic phosphorescence is also often observed inmolecules containing heteroatoms with unshared pairs of electrons atvery low temperatures. 2,2′-bipyridine is such a molecule. Non-radiativedecay mechanisms are typically temperature dependent, such that anorganic material that exhibits phosphorescence at liquid nitrogentemperatures typically does not exhibit phosphorescence at roomtemperature. But, as demonstrated by Baldo, this problem may beaddressed by selecting phosphorescent compounds that do phosphoresce atroom temperature. Representative emissive layers include doped orun-doped phosphorescent organo-metallic materials such as disclosed inU.S. Pat. Nos. 6,303,238 and 6,310,360; U.S. Patent ApplicationPublication Nos. 2002-0034656; 2002-0182441; 2003-0072964; andWO-02/074015.

Generally, the excitons in an OLED are believed to be created in a ratioof about 3:1, i.e., approximately 75% triplets and 25% singlets. See,Adachi et al., “Nearly 100% Internal Phosphorescent Efficiency In AnOrganic Light Emitting Device,” J. Appl. Phys., 90, 5048 (2001), whichis incorporated by reference in its entirety. In many cases, singletexcitons may readily transfer their energy to triplet excited states via“intersystem crossing,” whereas triplet excitons may not readilytransfer their energy to singlet excited states. As a result, 100%internal quantum efficiency is theoretically possible withphosphorescent OLEDs. In a fluorescent device, the energy of tripletexcitons is generally lost to radiationless decay processes that heat-upthe device, resulting in much lower internal quantum efficiencies. OLEDsutilizing phosphorescent materials that emit from triplet excited statesare disclosed, for example, in U.S. Pat. No. 6,303,238, which isincorporated by reference in its entirety.

Phosphorescence may be preceded by a transition from a triplet excitedstate to an intermediate non-triplet state from which the emissive decayoccurs. For example, organic molecules coordinated to lanthanideelements often phosphoresce from excited states localized on thelanthanide metal. However, such materials do not phosphoresce directlyfrom a triplet excited state but instead emit from an atomic excitedstate centered on the lanthanide metal ion. The europium diketonatecomplexes illustrate one group of these types of species.

Phosphorescence from triplets can be enhanced over fluorescence byconfining, preferably through bonding, the organic molecule in closeproximity to an atom of high atomic number. This phenomenon, called theheavy atom effect, is created by a mechanism known as spin-orbitcoupling. Such a phosphorescent transition may be observed from anexcited metal-to-ligand charge transfer (MLCT) state of anorganometallic molecule such as tris(2-phenylpyridine)iridium(III).

As used herein, the term “triplet energy” refers to an energycorresponding to the highest energy feature discernable in thephosphorescence spectrum of a given material. The highest energy featureis not necessarily the peak having the greatest intensity in thephosphorescence spectrum, and could, for example, be a local maximum ofa clear shoulder on the high energy side of such a peak.

The term “organometallic” as used herein is as generally understood byone of ordinary skill in the art and as given, for example, in“Inorganic Chemistry” (2nd Edition) by Gary L. Miessler and Donald A.Tarr, Prentice Hall (1998). Thus, the term organometallic refers tocompounds which have an organic group bonded to a metal through acarbon-metal bond. This class does not include per se coordinationcompounds, which are substances having only donor bonds fromheteroatoms, such as metal complexes of amines, halides, pseudohalides(CN, etc.), and the like. In practice, organometallic compoundsgenerally comprise, in addition to one or more carbon-metal bonds to anorganic species, one or more donor bonds from a heteroatom. Thecarbon-metal bond to an organic species refers to a direct bond betweena metal and a carbon atom of an organic group, such as phenyl, alkyl,alkenyl, etc., but does not refer to a metal bond to an “inorganiccarbon,” such as the carbon of CN or CO.

FIG. 1 shows an organic light emitting device 100. The figures are notnecessarily drawn to scale. Device 100 may include a substrate 110, ananode 115, a hole injection layer 120, a hole transport layer 125, anelectron blocking layer 130, an emissive layer 135, a hole blockinglayer 140, an electron transport layer 145, an electron injection layer150, a protective layer 155, and a cathode 160. Cathode 160 is acompound cathode having a first conductive layer 162 and a secondconductive layer 164. Device 100 may be fabricated by depositing thelayers described, in order.

Substrate 110 may be any suitable substrate that provides desiredstructural properties. Substrate 110 may be flexible or rigid. Substrate110 may be transparent, translucent or opaque. Plastic and glass areexamples of preferred rigid substrate materials. Plastic and metal foilsare examples of preferred flexible substrate materials. Substrate 110may be a semiconductor material in order to facilitate the fabricationof circuitry. For example, substrate 110 may be a silicon wafer uponwhich circuits are fabricated, capable of controlling OLEDs subsequentlydeposited on the substrate. Other substrates may be used. The materialand thickness of substrate 110 may be chosen to obtain desiredstructural and optical properties.

Anode 115 may be any suitable anode that is sufficiently conductive totransport holes to the organic layers. The material of anode 115preferably has a work function higher than about 4 eV (a “high workfunction material”). Preferred anode materials include conductive metaloxides, such as indium tin oxide (ITO) and indium zinc oxide (IZO),aluminum zinc oxide (AlZnO), and metals. Anode 115 (and substrate 110)may be sufficiently transparent to create a bottom-emitting device. Apreferred transparent substrate and anode combination is commerciallyavailable ITO (anode) deposited on glass or plastic (substrate). Aflexible and transparent substrate-anode combination is disclosed inU.S. Pat. Nos. 5,844,363 and 6,602,540 B2, which are incorporated byreference in their entireties. Anode 115 may be opaque and/orreflective. A reflective anode 115 may be preferred for sometop-emitting devices, to increase the amount of light emitted from thetop of the device. The material and thickness of anode 115 may be chosento obtain desired conductive and optical properties. Where anode 115 istransparent, there may be a range of thickness for a particular materialthat is thick enough to provide the desired conductivity, yet thinenough to provide the desired degree of transparency. Other anodematerials and structures may be used.

Hole transport layer 125 may include a material capable of transportingholes. Hole transport layer 130 may be intrinsic (undoped), or doped.Doping may be used to enhance conductivity. α-NPD and TPD are examplesof intrinsic hole transport layers. An example of a p-doped holetransport layer is m-MTDATA doped with F₄-TCNQ at a molar ratio of 50:1,as disclosed in U.S. patent application Publication No. 2002-0071963 A1to Forrest et al., which is incorporated by reference in its entirety.Other hole transport layers may be used.

Emissive layer 135 may include an organic material capable of emittinglight when a current is passed between anode 115 and cathode 160.Preferably, emissive layer 135 contains a phosphorescent emissivematerial, although fluorescent emissive materials may also be used.Phosphorescent materials are preferred because of the higher luminescentefficiencies associated with such materials. Emissive layer 135 may alsocomprise a host material capable of transporting electrons and/or holes,doped with an emissive material that may trap electrons, holes, and/orexcitons, such that excitons relax from the emissive material via aphotoemissive mechanism. Emissive layer 135 may comprise a singlematerial that combines transport and emissive properties. Whether theemissive material is a dopant or a major constituent, emissive layer 135may comprise other materials, such as dopants that tune the emission ofthe emissive material. Emissive layer 135 may include a plurality ofemissive materials capable of, in combination, emitting a desiredspectrum of light. Examples of phosphorescent emissive materials includeIr(ppy)₃. Examples of fluorescent emissive materials include DCM andDMQA. Examples of host materials include Alq₃, CBP and mCP. Examples ofemissive and host materials are disclosed in U.S. Pat. No. 6,303,238 toThompson et al., which is incorporated by reference in its entirety.Emissive material may be included in emissive layer 135 in a number ofways. For example, an emissive small molecule may be incorporated into apolymer. This may be accomplished by several ways: by doping the smallmolecule into the polymer either as a separate and distinct molecularspecies; or by incorporating the small molecule into the backbone of thepolymer, so as to form a co-polymer; or by bonding the small molecule asa pendant group on the polymer. Other emissive layer materials andstructures may be used. For example, a small molecule emissive materialmay be present as the core of a dendrimer.

Many useful emissive materials include one or more ligands bound to ametal center. A ligand may be referred to as “photoactive” if itcontributes directly to the luminescent properties of an organometallicemissive material. A “photoactive” ligand may provide, in conjunctionwith a metal, the energy levels from which and to which an electronmoves when a photon is emitted. Other ligands may be referred to as“ancillary.” Ancillary ligands may modify the photoactive properties ofthe molecule, for example by shifting the energy levels of a photoactiveligand, but ancillary ligands do not directly provide the energy levelsdirectly involved in light emission. A ligand that is photoactive in onemolecule may be ancillary in another. These definitions of photoactiveand ancillary are intended as non-limiting theories.

Electron transport layer 145 may include a material capable oftransporting electrons. Electron transport layer 145 may be intrinsic(undoped), or doped. Doping may be used to enhance conductivity. Alq₃ isan example of an intrinsic electron transport layer. An example of ann-doped electron transport layer is BPhen doped with Li at a molar ratioof 1:1, as disclosed in U.S. patent application Publication No.2002-0071963 A1 to Forrest et al., which is incorporated by reference inits entirety. Other electron transport layers may be used.

The charge carrying component of the electron transport layer may beselected such that electrons can be efficiently injected from thecathode into the LUMO (Lowest Unoccupied Molecular Orbital) energy levelof the electron transport layer. The “charge carrying component” is thematerial responsible for the LUMO energy level that actually transportselectrons. This component may be the base material, or it may be adopant. The LUMO energy level of an organic material may be generallycharacterized by the electron affinity of that material and the relativeelectron injection efficiency of a cathode may be generallycharacterized in terms of the work function of the cathode material.This means that the preferred properties of an electron transport layerand the adjacent cathode may be specified in terms of the electronaffinity of the charge carrying component of the ETL and the workfunction of the cathode material. In particular, so as to achieve highelectron injection efficiency, the work function of the cathode materialis preferably not greater than the electron affinity of the chargecarrying component of the electron transport layer by more than about0.75 eV, more preferably, by not more than about 0.5 eV. Similarconsiderations apply to any layer into which electrons are beinginjected.

Cathode 160 may be any suitable material or combination of materialsknown to the art, such that cathode 160 is capable of conductingelectrons and injecting them into the organic layers of device 100.Cathode 160 may be transparent or opaque, and may be reflective. Metalsand metal oxides are examples of suitable cathode materials. Cathode 160may be a single layer, or may have a compound structure. FIG. 1 shows acompound cathode 160 having a thin metal layer 162 and a thickerconductive metal oxide layer 164. In a compound cathode, preferredmaterials for the thicker layer 164 include ITO, IZO, and othermaterials known to the art. U.S. Pat. Nos. 5,703,436, 5,707,745,6,548,956 B2, and 6,576,134 B2, which are incorporated by reference intheir entireties, disclose examples of cathodes including compoundcathodes having a thin layer of metal such as Mg:Ag with an overlyingtransparent, electrically-conductive, sputter-deposited ITO layer. Thepart of cathode 160 that is in contact with the underlying organiclayer, whether it is a single layer cathode 160, the thin metal layer162 of a compound cathode, or some other part, is preferably made of amaterial having a work function lower than about 4 eV (a “low workfunction material”). Other cathode materials and structures may be used.

Blocking layers may be used to reduce the number of charge carriers(electrons or holes) and/or excitons that leave the emissive layer. Anelectron blocking layer 130 may be disposed between emissive layer 135and the hole transport layer 125, to block electrons from leavingemissive layer 135 in the direction of hole transport layer 125.Similarly, a hole blocking layer 140 may be disposed between emissivelayer 135 and electron transport layer 145, to block holes from leavingemissive layer 135 in the direction of electron transport layer 145.Blocking layers may also be used to block excitons from diffusing out ofthe emissive layer. The theory and use of blocking layers is describedin more detail in U.S. Pat. No. 6,097,147 and U.S. patent applicationPublication No. 2002-0071963 A1 to Forrest et al., which areincorporated by reference in their entireties.

As used herein, and as would be understood by one of skill in the art,the term “blocking layer” means that the layer provides a barrier thatsignificantly inhibits transport of charge carriers and/or excitonsthrough the device, without suggesting that the layer necessarilycompletely blocks the charge carriers and/or excitons. The presence ofsuch a blocking layer in a device may result in substantially higherefficiencies as compared to a similar device lacking a blocking layer.Also, a blocking layer may be used to confine emission to a desiredregion of an OLED.

Generally, injection layers are comprised of a material that may improvethe injection of charge carriers from one layer, such as an electrode oran organic layer, into an adjacent organic layer. Injection layers mayalso perform a charge transport function. In device 100, hole injectionlayer 120 may be any layer that improves the injection of holes fromanode 115 into hole transport layer 125. CuPc is an example of amaterial that may be used as a hole injection layer from an ITO anode115, and other anodes. In device 100, electron injection layer 150 maybe any layer that improves the injection of electrons into electrontransport layer 145. LiF / Al is an example of a material that may beused as an electron injection layer into an electron transport layerfrom an adjacent layer. Other materials or combinations of materialsmaybe used for injection layers. Depending upon the configuration of aparticular device, injection layers may be disposed at locationsdifferent than those shown in device 100. More examples of injectionlayers are provided in U.S. Pat. No. 7,071,615 which is incorporated byreference in its entirety. A hole injection layer may comprise asolution deposited material, such as a spin-coated polymer, e.g.,PEDOT:PSS, or it may be a vapor deposited small molecule material, e.g.,CuPc or MTDATA.

A hole injection layer (HIL) may planarize or wet the anode surface soas to provide efficient hole injection from the anode into the holeinjecting material. A hole injection layer may also have a chargecarrying component having HOMO (Highest Occupied Molecular Orbital)energy levels that favorably match up, as defined by theirherein-described relative ionization potential (EP) energies, with theadjacent anode layer on one side of the HIL and the hole transportinglayer on the opposite side of the HIL. The “charge carrying component”is the material responsible for the HOMO energy level that actuallytransports holes. This component may be the base material of the HIL, orit may be a dopant. Using a doped HIL allows the dopant to be selectedfor its electrical properties, and the host to be selected formorphological properties such as wetting, flexibility, toughness, etc.Preferred properties for the HIL material are such that holes can beefficiently injected from the anode into the HIL material. Inparticular, the charge carrying component of the HIL preferably has anIP not more than about 0.7 eV greater that the IP of the anode material.More preferably, the charge carrying component has an IP not more thanabout 0.5 eV greater than the anode material. Similar considerationsapply to any layer into which holes are being injected. HIL materialsare further distinguished from conventional hole transporting materialsthat are typically used in the hole transporting layer of an OLED inthat such HIL materials may have a hole conductivity that issubstantially less than the hole conductivity of conventional holetransporting materials. The thickness of the HIL of the presentinvention may be thick enough to help planarize or wet the surface ofthe anode layer. For example, an HIL thickness of as little as 10 nm maybe acceptable for a very smooth anode surface. However, since anodesurfaces tend to be very rough, a thickness for the HIL of up to 50 nmmay be desired in some cases.

A protective layer may be used to protect underlying layers duringsubsequent fabrication processes. For example, the processes used tofabricate metal or metal oxide top electrodes may damage organic layers,and a protective layer may be used to reduce or eliminate such damage.In device 100, protective layer 155 may reduce damage to underlyingorganic layers during the fabrication of cathode 160. Preferably, aprotective layer has a high carrier mobility for the type of carrierthat it transports (electrons in device 100), such that it does notsignificantly increase the operating voltage of device 100. CuPc, BCP,and various metal phthalocyanines are examples of materials that may beused in protective layers. Other materials or combinations of materialsmay be used. The thickness of protective layer 155 is preferably thickenough that there is little or no damage to underlying layers due tofabrication processes that occur after organic protective layer 160 isdeposited, yet not so thick as to significantly increase the operatingvoltage of device 100. Protective layer 155 may be doped to increase itsconductivity. For example, a CuPc or BCP protective layer 160 may bedoped with Li. A more detailed description of protective layers may befound in U.S. patent application Ser. No. Pat. No. 7,071,615 to Lu etal., which is incorporated by reference in its entirety.

FIG. 2 shows an inverted OLED 200. The device includes a substrate 210,an cathode 215, an emissive layer 220, a hole transport layer 225, andan anode 230. Device 200 may be fabricated by depositing the layersdescribed, in order. Because the most common OLED configuration has acathode disposed over the anode, and device 200 has cathode 215 disposedunder anode 230, device 200 may be referred to as an “inverted” OLED.Materials similar to those described with respect to device 100 may beused in the corresponding layers of device 200. FIG. 2 provides oneexample of how some layers may be omitted from the structure of device100.

The simple layered structure illustrated in FIGS. 1 and 2 is provided byway of non-limiting example, and it is understood that embodiments ofthe invention may be used in connection with a wide variety of otherstructures. The specific materials and structures described areexemplary in nature, and other materials and structures may be used.Functional OLEDs may be achieved by combining the various layersdescribed in different ways, or layers may be omitted entirely, based ondesign, performance, and cost factors. Other layers not specificallydescribed may also be included. Materials other than those specificallydescribed may be used. Although many of the examples provided hereindescribe various layers as comprising a single material, it isunderstood that combinations of materials, such as a mixture of host anddopant, or more generally a mixture, may be used. Also, the layers mayhave various sublayers. The names given to the various layers herein arenot intended to be strictly limiting. For example, in device 200, holetransport layer 225 transports holes and injects holes into emissivelayer 220, and may be described as a hole transport layer or a holeinjection layer. In one embodiment, an OLED may be described as havingan “organic layer” disposed between a cathode and an anode. This organiclayer may comprise a single layer, or may further comprise multiplelayers of different organic materials as described, for example, withrespect to FIGS. 1 and 2.

Structures and materials not specifically described may also be used,such as OLEDs comprised of polymeric materials (PLEDs) such as disclosedin U.S. Pat. No. 5,247,190, Friend et al., which is incorporated byreference in its entirety. By way of further example, OLEDs having asingle organic layer may be used. OLEDs may be stacked, for example asdescribed in U.S. Pat. No. 5,707,745 to Forrest et al, which isincorporated by reference in its entirety. The OLED structure maydeviate from the simple layered structure illustrated in FIGS. 1 and 2.For example, the substrate may include an angled reflective surface toimprove out-coupling, such as a mesa structure as described in U.S. Pat.No. 6,091,195 to Forrest et al., and/or a pit structure as described inU.S. Pat. No. 5,834,893 to Bulovic et al., which are incorporated byreference in their entireties.

Unless otherwise specified, any of the layers of the various embodimentsmay be deposited by any suitable method. For the organic layers,preferred methods include thermal evaporation, ink-jet, such asdescribed in U.S. Pat. Nos. 6,013,982 and 6,087,196, which areincorporated by reference in their entireties, organic vapor phasedeposition (OVPD), such as described in U.S. Pat. No. 6,337,102 toForrest et al., which is incorporated by reference in its entirety, anddeposition by organic vapor jet printing (OVJP), such as described inU.S. Pat. No. 7,431,968, which is incorporated by reference in itsentirety. Other suitable deposition methods include spin coating andother solution based processes. Solution based processes are preferablycarried out in nitrogen or an inert atmosphere. For the other layers,preferred methods include thermal evaporation. Preferred patterningmethods include deposition through a mask, cold welding such asdescribed in U.S. Pat. Nos. 6,294,398 and 6,468,819, which areincorporated by reference in their entireties, and patterning associatedwith some of the deposition methods such as ink-jet and OVJD. Othermethods may also be used. The materials to be deposited may be modifiedto make them compatible with a particular deposition method. Forexample, substituents such as alkyl and aryl groups, branched orunbranched, and preferably containing at least 3 carbons, may be used insmall molecules to enhance their ability to undergo solution processing.Substituents having 20 carbons or more may be used, and 3-20 carbons isa preferred range. Materials with asymmetric structures may have bettersolution processibility than those having symmetric structures, becauseasymmetric materials may have a lower tendency to recrystallize.Dendrimer substituents may be used to enhance the ability of smallmolecules to undergo solution processing.

The molecules disclosed herein may be substituted in a number ofdifferent ways without departing from the scope of the invention. Forexample, substituents may be added to a compound having three bidentateligands, such that after the substituents are added, one or more of thebidentate ligands are linked together to form, for example, atetradentate or hexadentate ligand. Other such linkages may be formed.It is believed that this type of linking may increase stability relativeto a similar compound without linking, due to what is generallyunderstood in the art as a “chelating effect.”

Devices fabricated in accordance with embodiments of the invention maybe incorporated into a wide variety of consumer products, including flatpanel displays, computer monitors, televisions, billboards, lights forinterior or exterior illumination and/or signaling, heads up displays,fully transparent displays, flexible displays, laser printers,telephones, cell phones, personal digital assistants (PDAs), laptopcomputers, digital cameras, camcorders, viewfinders, micro-displays,vehicles, a large area wall, theater or stadium screen, or a sign.Various control mechanisms may be used to control devices fabricated inaccordance with the present invention, including passive matrix andactive matrix. Many of the devices are intended for use in a temperaturerange comfortable to humans, such as 18 degrees C. to 30 degrees C., andmore preferably at room temperature (20-25 degrees C.).

The materials and structures described herein may have applications indevices other than OLEDs. For example, other optoelectronic devices suchas organic solar cells and organic photodetectors may employ thematerials and structures. More generally, organic devices, such asorganic transistors, may employ the materials and structures.

A compound comprising a carbene ligand bound to a metal center isprovided. Carbene compounds include small molecules, dendrimers, andpolymers that include a carbene-metal bond. In one embodiment, thecompound is a phosphorescent emissive material, preferably a dopant. Thecompound may also be doped into a wide band gap host material such asdisclosed in U.S. Pat. Publication No. 2004/0209116, which incorporatedby reference in its entirety, or it may be doped into an inert wide bandgap host such as disclosed in WO-074015, which is incorporated byreference in its entirety.

In another embodiment, the metal-carbene compound is a host material inan emissive layer. For example, the metal-carbene compound may be usedas a high energy host materials for doped blue devices. The dopant inthis case could be a triplet emitter or a singlet emitter (usingphosphor sensitized fluorescence). In some embodiments, the dopant is ablue or UV emissive material. In this case, the host material preferablyhas a wide energy gap. As used herein, the energy gap refers to thedifference in the energy between the highest occupied molecular orbital(HOMO) and the lowest unoccupied molecular orbital (LUMO) for aparticular compound. The triplet energy for a given material is relatedto, but less than, the energy gap. Materials for use as a wide gap hostare selected to have a wide energy gap so that the host material doesnot quench the dopant emission by endothermic or exothermic energytransfer. The wide gap host is preferably selected so as to have atriplet energy at least about 300 mV higher than that of the dopant.

Additionally, the high band gap of metal-carbene compounds may makethese materials effective in carrier blocking and transporting layers.Specifically, these materials may be used in the electron blockinglayer, hole blocking layer, exciton blocking layer, hole transportlayer, or electron transport layer of an OLED. In other embodiments ametal-carbene compound may be used as a hole injection layer, electroninjection layer, or protective layer. It is believed that metal-carbenecompounds described herein have improved thermal stability whenincorporated into an organic light emitting device due to thecarbene-metal bond, as compared to existing compounds without acarbene-metal bond.

As used herein, the term “carbene” refers to compounds having a divalentcarbon atom with only six electrons in its valence shell when notcoordinated to a metal. A useful exercise to determine whether a ligandincludes a carbene-metal bond is to mentally deconstruct the complex asa metal fragment and a ligand, and to then determine whether a carbonatom in the ligand that was previously bound to the metal is a neutraldivalent carbon atom in the deconstructed state. The resonance forms ofa preferred embodiment may be shown as:

This definition of carbene is not limited to metal-carbene complexessynthesized from carbenes, but is rather intended to address the orbitalstructure and electron distribution associated with the carbon atom thatis bound to the metal. The definition recognizes that the “carbene” maynot technically be divalent when bound to the metal, but it would bedivalent if it were detached from the metal. Although many suchcompounds are synthesized by first synthesizing a carbene and thenbinding it to a metal, the definition is intended to encompass compoundssynthesized by other methods that have a similar orbital structure andelectron configuration. Lowry & Richardson, Mechanism and Theory inOrganic Chemistry 256 (Harper & Row, 1976) defines “carbene” in a waythat is consistent with the way the term is used herein. Some referencesmay define “carbene” as a carbon ligand that forms a double bond to ametal. While this definition is not being used in the presentapplication, there may be some overlap between the two definitions. Avariety of representations are used to depict the bonding in suchcarbenes, including those in which a curved line is used to indicatepartial multiple bonding between the carbene carbon and the adjacentheteroatom(s).

In the figures and structures herein, a carbene-metal bond may bedepicted as C→M, as for example:

Such structures that use an arrow to represent the presence of ametal-carbene bond are used interchangeably herein with structures thatdo not include the arrow, without any intention of suggesting there is adifference in the structure shown.

Carbene ligands are especially desirable in OLED applications due to thehigh thermal stability exhibited by metal-carbene complexes. It isbelieved that the carbene, which behaves much as an electron donativegroup, generally bonds strongly to the metals, thus forming a morethermally stable complex than, for example, previous cyclometallatedcomplexes used as phosphorescent emitters. It is also believed thatcarbene analogs of ligands employed in existing phosphorescent emissivematerials (for example the phenylpyridine or Irppy, etc.) may exhibitgreater stability and emit at substantially higher energy than theirexisting analogs.

As used herein, a “non-carbene analog” of a metal carbene compoundrefers to existing ligands having a substantially similar chemicalstructure to the metal-carbene compound, but unlike the carbenecompounds of the present invention, which features a carbene-metal bond,the analog has some other bond, such as a carbon-metal or anitrogen-metal bond, in place of the carbene-metal bond. For example,Ir(ppz)₃ has a nitrogen in each ligand bound to the Ir.Ir(1-phenylimidazolin-2-ylidene) is analogous to Ir(Ppz)₃ where thenitrogen bound to the Ir has been replaced with a carbene bound to theIr, and where the atoms surrounding the carbene have been changed tomake the carbon a carbene. Thus, embodiments of the present inventioninclude metal-carbene complexes (e.g. Ir(1-phenylimidazolin-2-ylidene)with similar structures to existing emissive compounds (e.g. Ir(ppz)₃).

Examples of existing emissive compounds include Ir(ppy)₃ and Ir(ppz)₃,discussed above. Other examples are disclosed in the references below,which are incorporated herein by reference in their entirety. Inpreferred embodiments, the carbene ligands are imidazoles, pyrazoles,benzimidazoles, and pyrroles.

It is believed that the carbene-metal bond in Ir(1-Ph-3-Me-imid)₃ isstronger than the N-metal bond in Ir(ppz)₃. Moreover, due to the natureof a carbene-metal bond, it is believed that replacing a carbon-metalbond or nitrogen-metal bond in existing emissive organometallicmolecules with a carbene-metal bond (making other changes as needed tomake the carbon atom a carbene) may result in an emissive molecule thatis more stable than the non-carbene analog, and that has strongerspin-orbit coupling. It is further believed that the emissive spectra ofthe molecule including a carbene may be different from the emissivespectra of the analog without a carbene.

Metal-carbene complexes may be tuned to emit a wide variety of spectrafrom the near-ultraviolet across the entire visible spectra by theselection of substituents and/or chemical groups on the ligand(s). Moresignificantly, it may now be possible to obtain saturated blue coloremissions with peak wavelengths at about 450 nm. Because it is believedto be materially easier to reduce than to increase the triplet energy bytuning an emissive compound, the ability to make stable blue emitters atsuch high energies would also allow for the possibility of obtaining anycolor by reducing the energy so as to red-shift the emission.

The appropriate selection of substituents and/or chemical groupsattached to carbene ligands may also minimize quantum efficiency lossesassociated with increasing temperatures. The observable difference inlifetime measurements between emission at room temperature and at lowtemperatures (e.g. 77 K) is believed to be attributed to non-radiativequenching mechanisms that compete with phosphorescent emission. Suchquenching mechanisms are further believed to be thermally activated, andconsequently, at cooler temperatures of about 77 K, where energy lossdue to quenching is not an issue, quantum efficiency is about 100%. Itis believed that appropriate substituents on the carbene ligand, ordoping in a more rigid matrix, such as disclosed in Turro, “ModemMolecular Photochemistry”, University Science Books (1991), 109-10, mayincrease quantum efficiency at room temperature and correspondingly showlonger lifetimes.

Due to the nature of the carbene-metal bond, the emission of a carbeneanalog may be substantially different from that of its non-carbeneanalog, and the emission of the carbene analog may be stable and at ahigher energy than previously obtainable with stable non-carbenecompounds.

In some embodiments, the triplet energy of the carbene complex has acorresponding wavelength in the deep blue or ultraviolet (UV) part ofthe spectra. In some embodiments, the phosphorescent emissive compoundhas triplet energy corresponding to a wavelength of less than 450 nm. Inpreferred embodiments, the triplet energy corresponds to a wavelength ofless than 440 nm, and in even more preferred embodiments, it correspondsto a wavelength less than 400 nm, which is believed to be in the UVregion of the spectrum, since 400 nm is believed to represent thecut-off between the UV and the visible regions of the spectrum. Suchhigh triplet energy may make these compounds useful in optically pumpingdown converting layers. For such applications, an overlap is preferredbetween the emission spectra of the ultraviolet carbene compound and theabsorption spectra of the down converting layer. It is believed thatwhen about 50% of the integral of the curve for the normalizedelectroluminescent spectra of the device is at a wavelength less thanabout 450 nm, there is sufficient energy to optically pump a downconverting layer. More preferably, greater than 90% of the emission maybe produced below 440 nm, as disclosed herein. Preferably, 50% of theintegral of the curve for the normalized electroluminescent spectra isless than about 440 nm, and more preferably, it is less than about 400nm. The wavelength cutoffs mentioned above are not intended to beabsolute limitations as they depend on the energy of the material to bepumped. It is also believed that these emissions may occur at roomtemperature.

The strong metal-carbon bond is also believed to lead to greaterspin-orbit coupling in metal carbene complexes. Moreover, the tripletenergy of coordinated carbenes are shown to be significantly higher thanpyridine analogs.

The stability of metal-carbene complexes may also allow increasedversatility in the types of ligands and metals that may be used asphosphorescent emitters in OLEDs. The strong metal-carbene bond mayallow a variety of metals to form useful phosphorescent complexes withcarbene ligands to give novel emissive compounds. For example, oneembodiment includes gold or copper bonded to a carbene ligand. Suchmetals have been calculated to form metal-carbon bonds having quite highbond dissociation energies, such as illustrated in Nemcsok et al., “TheSignificance of πr-Interactions in Group 11 Complexes withN-Heterocyclic Carbenes,” American Chemical Society, Publ. on Web, Jun.19, 2004. Such high bond dissociation energies may be expected toimprove the chemical stability of metal-carbene complexes as comparedwith the analogous metal-phenyl-pyridine (“metal-ppy”) based complexesthat are typically used in an OLED. Thus, in addition to their use asthe emissive materials in an OLED, metal-carbene complexes may be alsoused advantageously, because of their improved chemical stability, forother functions in an OLED, for example, as a host material in theemissive layer, as an electron or hole transporting material in anelectron or hole transporting layer, and/or as an electron or holeblocking material in an electron or hole blocking layer.

Additionally, although cyclometallated complexes are preferredembodiments, the present invention is not limited to such embodiments.The increased strength of a metal-carbene bond, as compared to othertypes of bonds with metal, may make monodentate ligands feasible for useas emissive materials. Until recently, bidentate ligands were stronglypreferred due to stability concerns. Thus, embodiments includemonodentate carbene ligands as well as bidentate. Embodiments alsoinclude tridentate carbene ligands, which may be quite stable, and manyexamples are found in the art, such as those disclosed in Koizumi etal., Organometallics 2003, 22, 970-975. Other embodiments may alsofeature a tetradentate ligand, such as porphyrin analogs in which one ormore nitrogens are replaced by a carbene, which is disclosed inBourissou et al. Chem Rev. 2000, 100, 39-9 1. Still other embodimentsmay include metallaquinone carbenes, which are compounds in which one ofthe oxygen atoms of a quinone has been replaced by a metal, such asthose discbsed in Ashekenazi et al., J. Am. Chem. Soc. 2000, 122,8797-8798. In addition, the metal-carbene compound may be present aspart of a multi-dentate group such as disclosed in U.S. Pat. PublicationNo. 2005-0170206 Al , which is incorporated by reference in itsentirety.

It is believed that many of the (C,C) or (C,N) ligands of many existingelectroluminescent compounds may be modified to create an analogous(C,C) ligand including a carbene. Specific non limiting examples of suchmodification include:

-   -   (1) the substituents on the carbene-bonded branch of the        (C,C)-ligand and the substituents on the        mono-anionic-carbon-bonded branch of the (C,C)-ligand may be        independently selected from the group consisting of        -   (a) the substituents on the N-bonded branch of the existing            (C,N)-ligands, such as disclosed in the references listed            below, which is typically but not necessarily a pyridine            group; and        -   (b) the substituents on the mono-anionic-carbon-bonded            branch of the existing (C,N)-ligands, such as disclosed in            the references listed below, which is typically but not            necessarily a phenyl group;        -   (c) and/or a combination thereof; and    -   (2) the compounds including the metal-carbene bonds may further        include ancillary ligands selected from the group consisting of        the ancillary ligands such as disclosed in the following        references:        U.S. patent application Publ. No. 2002-0034656, FIGS. 11-50,        U.S. patent application Publ. No. 2003-0072964 (Thompson et        al.), paragraphs 7-132; and FIGS. 1-8; U.S. patent application        Publ. No. 2002-0182441 (Lamansky et al.), paragraphs 13-165,        including FIGS. 1-9 (g); U.S. Pat. No. 6,420,057 B1 (Ueda et        al.), col. 1, line 57, through col. 88, line 17, including each        compound I-1 through XXIV-12; U.S. Pat. No. 6,383,666 B1 (Kim et        al.), col. 2, line 9, through col. 21, lin3 67; U.S. patent        application Publ. No. 2001-0015432 A1 (Igarashi et al.),        paragraphs 2-57, including compounds (1-1) through (1-30); U.S.        patent application Publ. No. 2001-0019782 A1 (Igarashi et al.),        paragraphs 13-126, including compounds (1-1) through (1-70), and        (2-1) through (2-20); U.S. patent application Publ. No.        2002-0024293 (Igarashi et al.), paragraphs 7-95, including        general formulas K-I through K-VI, and example compounds (K-1)        through (K-25); U.S. patent application Publ. No. 2002-0048689        A1 (Igarashi et al.), paragraphs 5-134, including compounds        1-81, and example compounds (1-1) through (1-81); U.S. patent        application Publ. No. 2002-0063516 (Tsuboyama et al.),        paragraphs 31-161, including each compound 1-16; U.S. patent        application Publ. No. 2003-0068536 (Tsuboyama et al.),        paragraphs 31-168, including each compound in Tables 1-17,        corresponds to EP-1-239-526-A2; U.S. patent application Publ.        No. 2003-0091862 (Tokito et al.), paragraphs 10-190, including        each compound in Tables 1-17, corresponds to EP-1-239-526-A2;        U.S. patent application Publ. No. 2003-0096138 (Lecloux et al.),        paragraphs 8-124, including FIGS. 1-5; U.S. patent application        Publ. No. 2002-0190250 (Grushin et al.), paragraphs 9-191; U.S.        patent application Publ. No. 2002-0121638 (Grushin et al.),        paragraphs 8-125; U.S. patent application Publ. No. 2003-0068526        (Kamatani et al.), paragraphs 33-572, including each compound in        Tables 1-23; U.S. patent application Publ. No. 2003-0141809        (Furugori et al.), paragraphs 29-207; U.S. patent application        Publ. No. 2003-0162299 A1 (Hsieh et al.), paragraphs 8-42; WO        03/084972, (Stossel et al.), Examples 1-33; WO 02/02714 A2        ((Petrov et al.), pages 2-30, including each compound in Tables        1-5; EP 1-191-613 A1(Takiguchi et al.), paragraphs 26-87,        including each compound in Tables 1-8, (corresponding to U.S.        patent application Publ. No. 2002-0064681); and EP 1-191-614 A2        (Tsuboyama et al.), paragraphs 25-86, including each compound in        Tables 1-7; which are incorporated herein by reference in their        entirety.

Carbene ligands may be synthesized using methods known in the art, suchas those disclosed in Cattoën, et al., J. Am. Chem. Soc., 2004, 126;1342-1343; Chiu-Yuen Wong, et al, Organometallics 2004, 23, 2263-2272;Klapars, et al, J. Am. Chem. Soc., 2001, 123; 7727-7729; Bourissou etal. Chem Rev. 2000, 100,39-91; Siu-Wai Lai, et al, Organometallics 1999,18, 3327-3336; Wen-Mei Xue et al., Organometallics 1998, 17, 1622-1630;Wang & Lin, Organometallics 1998, 17, 972-975; Cardin, et al., Chem Rev.1972, 5, 545-574; and other references discussed herein.

The term “halo” or “halogen” as used herein includes fluorine, chlorine,bromine and iodine.

The term “alkyl” as used herein contemplates both straight and branchedchain alkyl radicals. Preferred alkyl groups are those containing fromone to fifteen carbon atoms and includes methyl, ethyl, propyl,isopropyl, butyl, isobutyl, tert-butyl, and the like. Additionally, thealkyl group may be optionally substituted with one or more substituentsselected from halo, CN, CO₂R, C(O)R, NR₂, cyclic-amino, NO₂, and OR.

The term “cycloalkyl” as used herein contemplates cyclic alkyl radicals.Preferred cycloalkyl groups are those containing 3 to 7 carbon atoms andincludes cyclopropyl, cyclopentyl, cyclohexyl, and the like.Additionally, the cycloalkyl group may be optionally substituted withone or more substituents selected from halo, CN, CO₂R, C(O)R, NR₂,cyclic-amino, NO₂, and OR.

The term “alkenyl” as used herein contemplates both straight andbranched chain alkene radicals. Preferred alkenyl groups are thosecontaining two to fifteen carbon atoms. Additionally, the alkenyl groupmay be optionally substituted with one or more substituents selectedfrom halo, CN, CO₂R, C(O)R, NR₂, cyclic-amino, NO₂, and OR.

The term “alkynyl” as used herein contemplates both straight andbranched chain alkyne radicals. Preferred alkyl groups are thosecontaining two to fifteen carbon atoms. Additionally, the alkynyl groupmay be optionally substituted with one or more substituents selectedfrom halo, CN, CO₂R, C(O)R, NR₂, cyclic-amino, NO₂, and OR.

The terms “aralkyl” as used herein contemplates an alkyl group that hasas a substituent an aromatic group. Additionally, the aralkyl group maybe optionally substituted on the aryl with one or more substituentsselected from halo, CN, CO₂R, C(O)R, NR₂, cyclic-amino, NO₂, and OR.

The term “heterocyclic group” as used herein contemplates non-aromaticcyclic radicals. Preferred heterocyclic groups are those containing 3 or7 ring atoms which includes at least one hetero atom, and includescyclic amines such as morpholino, piperdino, pyrrolidino, and the like,and cyclic ethers, such as tetrahydrofuran, tetrahydropyran, and thelike.

The term “aryl” or “aromatic group” as used herein contemplatessingle-ring groups and polycyclic ring systems. The polycyclic rings mayhave two or more rings in which two carbons are common by two adjoiningrings (the rings are “fused”) wherein at least one of the rings isaromatic, e.g., the other rings can be cycloalkyls, cycloalkenyls, aryl,heterocycles and/or heteroaryls.

The term “heteroaryl” as used herein contemplates single-ringhetero-aromatic groups that may include from one to four heteroatoms,for example, pyrrole, furan, thiophene, imidazole, oxazole, thiazole,triazole, tetrazole, pyrazole, pyridine, pyrazine and pyrimidine, andthe like. The term heteroaryl also includes polycyclic hetero-aromaticsystems having two or more rings in which two atoms are common to twoadjoining rings (the rings are “fused”) wherein at least one of therings is a heteroaryl, e.g., the other rings can be cycloalkyls,cycloalkenyls, aryl, heterocycles and/or heteroaryls.

All value ranges, for example those given for n and m, are inclusiveover the entire range. Thus, for example, a range between 0-4 wouldinclude the values 0, 1, 2, 3 and 4.

Embodiments include photoactive carbene ligands. m represents the numberof photoactive ligands. For example, for Ir, m may be 1, 2 or 3. n, thenumber of “ancillary” ligands of a particular type, may be any integerfrom zero to one less than the maximum number of ligands that may beattached to the metal. (X—Y) represents an ancillary ligand. Thedefinitions of photoactive and ancillary are intended as non-limitingtheories. For example, for Ir, n may be 0, 1 or 2 for bidentate ligands.Ancillary ligands for use in the emissive material may be selected fromthose known in the art. Non-limiting examples of ancillary ligands maybe found in PCT Application Publication WO 02/15645 A1 to Lamansky etal. at pages 89-90, which is incorporated herein by reference.

The metal forming the metal-carbene bond may be selected from a widerange of metals. Preferred metals include main group metals, 1^(st) rowtransition metals, 2^(nd) row transition metals, 3^(rd) row transitionmetals, and lanthanides. Although one skilled in the art typicallyexpects room temperature phosphorescence only from metal atoms thatexert a strong heavy atom effect, phosphorescent emission has beenobserved in Kunkley, et al. J. Organometallic Chem. 2003, 684, 113-116for a compound with a Nickel (Ni) metal, which is typically not expectedto exert a strong heavy atom effect. Thus, embodiments also includefirst row transition metal, such as Ni, and other metals that do notnormally exert a strong heavy atom effect but exhibits phosphorescentemission when coordinated to one or more carbene ligands. More preferredmetals include 3^(rd) row transition metals. The following are alsopreferred metals: Ir, Pt, Pd, Rh, Re, Ru, Os, Tl, Pb, Bi, In, Sn, Sb,Te, Au, and Ag. Most preferably, the metal is Iridium.

The most preferred embodiments are N-heterocyclic carbenes, whichBourissou has also reported as having “remarkable stability” as freecompounds in Bourissou et al. Chem Rev. 2000, 100, 39-91.

Preferred embodiments include metal-carbene compounds having the generalstructure selected from:

with corresponding ligands having the structures selected from

in which R₁ may be hydrogen, alkyl, ailcenyl, alkynyl, aralkyl, aryl,heteroaryl, substituted aryl, substituted heteroaryl, or a heterocyclicgroup; R_(2,) R₃, R₈, R₁₀, R₁₁ and R₁₂ may independently be hydrogen,alkyl, alkenyl, alkynyl, aralkyl, CN, CF₃, CO₂R′, C(O)R′, C(O)NR′₂,NR′₂NO2, OR′, SR′, SO₂, SOR′, SO₃R′, halo, aryl, heteroaryl, substitutedaryl, substituted heteroaryl, or a heterocyclic group; and additionallyor alternatively, one or more of R₁ and R₂, and R₂ and R₃, and two R₈groups, two R₁₀ groups, and two R₁₂ groups on the same ring together mayfonn independently a 5 or 6-member cyclic group, wherein said cyclicgroup is cycloalkyl, cycloheteroalkyl, aryl or heteroaryl; and whereinsaid cyclic group may be optionally substituted by one or moresubstituents J; each substituent J may be R′, CN, CF₃, C(O)OR′, C(O)R′,C(O)NR′₂, NR′₂, NO₂, OR′, SR.′, SO₂, SOR′, or SO₃R′, and additionally,or alternatively, two J groups on adjacent ring atoms may form a fused5- or 6-membered aromatic group; each R′may be independently halo, H,alkyl, alkenyl, alkynyl, heteroalkyl, aralkyl, aryl and heteroaryl;(X-Y) may be a photoactive ligand or an ancillary ligand, d is 0, 1, 2,3, or4;m is a value from 1 to the maximum number of ligands that may beattached to metal M;m +n is the maximum number of ligands that may be attached to metal M.

In a most preferred embodiment, the metal is Ir. Preferably, m is 3 andn is 0. In one embodiment, R6 is methyl. In another embodiment m is 2and n is one. The ancillary ligand X—Y may have one of the followingstructures:

Other preferred ancillary ligands are acetylacetonate, picolinate, andpyrazolyls (as found in the reference Li, J. et al. Polyhedron 2004, 23,419-428), and their derivatives.

More preferred embodiments have the following structures:

and more preferred ligands have the following corresponding structures

Devices incorporating the above compounds have been shown to exhibitphosphorescent emissions that span the spectrum of blue light. Forexample, Dopant A and B, in Examples 6 and 7 respectively, are UVemitters in solution photoluminescence (PL) and the electroluminescence(EL) emission has a highest energy peak or shoulder at about 400 nm asshown in FIG. 5. Dopants M, N, O, and P, in Examples 18, 19, 20 and 21respectively, have slightly lower triplet energy and emit in a devicewith high efficiency using the wide band gap (high triplet energy) host,UGH5. These examples have similar triplet energy and the EL emissionshows vibronic fine structure characteristic of the dopant. The emissionfrom these examples is a desirable blue color with a CIE coordinate ofabout (0.14, 0.15).

Other devices, for example, the devices of Examples 13, 14 and 15(Dopants H, I, and J) exhibit emissions at lower energy with a highesttriplet energy emission at about 470 nm and vibronic fine structurecharacteristic of the dopant (FIG. 17). Examples 8 and 9 (Dopants C andD) exhibit still lower energy with highest energy emission at about 485nm (FIG. 8).

It is understood that the various embodiments described herein are byway of example only, and are not intended to limit the scope of theinvention. For example, many of the materials and structures describedherein may be substituted with other materials and structures withoutdeviating from the spirit of the invention. It is understood thatvarious theories as to why the invention works are not intended to belimiting. For example, theories relating to charge transfer are notintended to be limiting.

MATERIAL DEFINITIONS

As used herein, abbreviations refer to materials as follows: CBP:4,4′-N,N-dicarbazole-biphenyl m-MTDATA 4,4′,4″-tris(3-methylphenylphenlyamino)triphenylamine Alq₃: 8-tris-hydroxyquinolinealuminum Bphen: 4,7-diphenyl-1,10-phenanthroline n-BPhen: n-doped BPhen(doped with lithium) F₄-TCNQ: tetrafluoro-tetracyano-quinodimethanep-MTDATA: p-doped m-MTDATA (doped with F₄- TCNQ) Ir(ppy)₃:tris(2-phenylpyridine)-iridium Ir(ppz)₃: tris(1-phenylpyrazoloto,N,C(2′)iridium(III) BCP: 2,9-dimethyl-4,7-diphenyl-1,10- phenanthrolineTAZ: 3-phenyl-4-(1′-naphthyl)-5-phenyl-1,2,4- triazole CuPc: copperphthalocyanine. ITO: indium tin oxide NPD:N,N′-diphenyl-N-N′-di(1-naphthyl)- benzidine TPD:N,N′-diphenyl-N-N′-di(3-toly)-benzidine BAlq, BAlQ:aluminum(III)bis(2-methyl-8- hydroxyquinolinato)4-phenylphenolate mCP:1,3-N,N-dicarbazole-benzene DCM: 4-(dicyanoethylene)-6-(4-dimethylaminostyryl-2-methyl)-4H-pyran DMQA: N,N′-dimethylquinacridonePEDOT:PSS: an aqueous dispersion of poly(3,4- ethylenedioxythiophene)with polystyrenesulfonate (PSS) UGH5 1,3-bis(triphenylsilyl)benzene mCBP3,3′-N,N-dicarbazole-biphenyl 1-Ph-3-Me-imid1-phenyl-3-methyl-imidazolin-2-ylidene- C,C^(2′) mer-Ir(1-Ph-3-mer-iridium(III)tris(1-phenyl-3-methyl- Me-imid)₃imidazolin-2-ylidene-C,C^(2′))

EXPERIMENTAL

Specific representative embodiments of the invention will now bedescribed, including how such embodiments may be made. It is understoodthat the specific methods, materials, conditions, process parameters,apparatus and the like do not necessarily limit the scope of theinvention.

EXAMPLE 1 Synthesis of fac-iridium(III)tris(n-[p-trimethylsilylphenyl]-2-methyl-benzimidazole) Step 1:Synthesis of N-(p-trimethylsilylphenyl)benzimidazole

To a 1L 3-neck flask equipped with a mechanical stir arm was added 14.5g (65.1 mmol) of 1-bromo-4-trimethylsilylbenzene, 9.2 g (78.1 mmol) ofbenzimidazole, 9.1 g (143 mmol) of copper power, 31.4 g (228 mmol) ofpotassium carbonate, and 1.7 g (6.5 mmol) of 18-crown-6. These chemicalswere stirred vigorously in 400 mL tetrahydronaphthalene at 180° C. underN₂ atmosphere. After 20 hours of heating, the mixture was filtered warm.The solids on the funnel were repeatedly washed with dichloromethane toremove all organic products as filtrate. The mother liquor was thenevaporated in vacuo and pooled with a small batch of crude material froma previous reaction. This residue was purified by distilling on aKugelrohr apparatus twice. At 160° C., the removal of organic impuritieswas evidenced. At 200° C. the product distilled. Purification gave 14.2g N-(p-trimethylsilylphenyl)benzimidazole as a white solid.

Step 2: Methylation of N-(p-trimethylsilylphenyl)benzimidazole

14.1 g (52.9 mmol) of N-(p-trimethylsilylphenyl)benzimidazole wassolubilized with 200 mL of toluene in a 500 mL flask equipped with astir bar. To the stirred solution was carefully added 22.5 g (159 mmol)iodomethane and then was allowed to reflux for 2 hours. Next, thecondenser was removed and the flask enriched with 100 mL toluene. Astream of N₂ was then passed over the flask to remove excessiodomethane. After the volume of the solution was reduced by half, itwas cooled and the white solids collected on a filter and rinsed withtoluene followed by hexanes. 20.8 gN-(p-trimethylsilylphenyl)benzimidazole was achieved after drying invacuo (96.3% yield).

Step 3: Synthesis of fac-iridium(III)tris(n-[p-trimethylsilylphenyl]-2-methyl-benzimidazole)

20 mL 2-methoxyethanol was purged with N₂ and refluxed in a 100 mL roundbottom flask equipped with a stir bar. After deoxygenation of thesolvent, the flask was cooled and 1.0 g (2.45 mmol)N-(p-trimethylsilylphenyl)benzimidazole was added along with 0.29 g(0.29 mmol) IrCl₃.3H₂O and 0.76 g (3.27 mmol) silver(I) oxide. Themixture was refluxed for 2 hours under a stream of N₂. The reactionmixture was cooled and the solvent evaporated in vacuo. This residue wasdissolved in a minimal amount of dichloromethane and purified on asilica gel column using dichloromethane as eluent. The product fractionswere evaporated of solvent and recrystallized from methylenechloride/methanol to give 0.14 g fac-iridium(III)tris(n-[p-trimethylsilylphenyl]-2-methyl-benzimidazole) as pale-greysolids (MS confirmed)

EXAMPLES 2, 3 Synthesis of fac(mer)-iridium(III)tris(n-[3-biphenyl]-2-methyl-benzimidazole) [Dopant E, F] Step 1:Synthesis of N-(3-biphenyl)benzimidazole

To a 3 L 3-neck flask equipped with a mechanical stir arm was added 100g (429 mmol) of 3-bromobiphenyl, 60.7 g (514 mmol) of benzimidazole, 60g (943 mmol) of copper powder, 207 g (1500 mmol) of potassium carbonateand 11.3 g (42.9 mmol) of 18-crown-6. These chemicals were stirredvigorously in ˜1.5 L tetrahydronaphthalene at 180° C. under N₂atmosphere. After 48 hours of heating, the mixture was filtered warm.The solids on the funnel were repeatedly washed with dichloromethane toremove all organic products as filtrate. The mother liquor was thenevaporated in vacuo to give a dark residue that was solubilized with aminimal amount of dichloromethane and purified on a silica gel columnusing a gradient of 50% EtOAc/hexanes→100% EtOAc. The purest fractionswere evaporated of solvent, and the resultant solids sonciated inhexanes, filtered, washed with hexanes and dried to give 74 gN-(3-biphenyl)benzimidazole as off-white solids (64% yield).

Step 2: Methylation of N-(3-biphenyl)-benzimidazole

74 g (274 mmol) of N-(3-biphenyl)benzimidazole was solubilized with 400mL of toluene in a 1 L flask equipped with a stir bar. To the stirredsolution was carefully added 117 g (821 mmol) iodomethane and then wasallowed to reflux for 2 hours. Next, the condenser was removed and theflask enriched with 200 mL toluene. A stream of N₂ was then passed overthe flask to remove excess iodomethane. After the volume of the solutionwas reduced by half, it was cooled and the white solids collected on afilter and rinsed with toluene followed by hexanes. 108 gN-(3-biphenyl)-2-methylbenzimidazole was achieved after drying in vacuo(95.6% yield).

Step 3: Synthesis of fac(mer)-iridium(III)tris(n-[3-biphenyl]-2-methyl-benzimidazole)

200 mL 2-methoxyethanol was purged with N₂ and refluxed in a 500 mLround bottom flask equipped with a stir bar. After deoxygenation of thesolvent, the flask was cooled and 10.0 g (24.3 mmol)N-(3-biphenyl)-2-methylbenzimidazole was added along with 2.18 g (6.06mmol) IrCl₃.3H₂O and 4.2 g (18.2 mmol) silver(I) oxide. The mixture wasrefluxed for 30 minutes whereupon an additional 4.2 g (18.2 mmol)silver(I) oxide was added. After heating for an additional 1.5 hours atreflux, HPLC indicated favorable formation of both the mer and facisomers (˜1:2 ratio). The reaction mixture was cooled, enriched with 100mL dichloromethane and filtered. Whereas the filtrate was put to theside (mer-rich), the resulting solids were removed the filter, stirredvigorously in 500 mL MeCl₂ and filtered again. This process wasperformed a second time and the 1 L of dichloromethane filtrates wereevaporated of solvent to give beige solids that were 1× recrystallizedfrom dichloromethane/methanol giving 0.95 g fac-iridium(III)tris(n-[3-biphenyl]-2-methyl-benzimidazole) as off-white solids. Thismaterial was sublimed in vacuo and afforded 0.23 g to be tested in anOLED device (99.5% HPLC assay, ¹HNMR confirmed). The mer-rich filtratewas evaporated of solvent and the residue solubilized in dichloromethaneand purified on a column of silica using dichloromethane as eluent. Themer-rich fractions were evaporated of solvent and recrystallized fromacetonitrile to give ˜1.0 g 4:1 mer/fac iridium(III)tris(n-[3-biphenyl]-2-methyl-benzimidazole) as an off-white solid thatcould not be sublimed (¹H NMR confirmed).

EXAMPLES 4, 5 Synthesis of fac(mer)-iridium(III)tris(1-(N-[2-methylbenzimidazole])-4-(o-tolyl)benzene) [Dopant G] Step1: Synthesis of 1-(N-benzimidazole)-4-(o-tolyl)benzene

To a 2 L 3-neck flask equipped with a mechanical stir arm was added 46.6g (189 mmol) of 1-bromo-4-(o-tolyl)benzene, 26.7 (226 mmol) ofbenzimidazole, 26.3 g (415 mmol) of copper powder, 91.1 g (660 mmol) ofpotassium carbonate and 5.0 g (18.9 mmol) of 18-crown-6. These chemicalswere stirred vigorously in ˜1.0 L tetrahydronaphthalene at 180° C. underN₂ atmosphere. After 48 hours of heating, the mixture was filtered warm.The solids on the funnel were repeatedly washed with dichloromethane toremove all organic products as filtrate. The mother liquor was thenevaporated in vacuo to give a dark residue that was solubilized with aminimal amount of dichloromethane and purified on a silica gel columnusing a gradient of 50% EtOAc/hexanes→100% dicoloromethane. The purestfractions were evaporated of solvent and the resultant residue distilledvia a Kugelrohr apparatus (190° C.) to give 26.1 g1-(N-benzimidazole)-4-(o-tolyl)benzene as a clear liquid (48.5% yield).

Step2: Methylation of 1-(N-benzimidazole)-4-(o-tolyl)benzene

26.1 g (91.8 mmol) of 1-(N-benzimidazole)-4-(o-tolyl)benzene wassolubilized with 400 mL of toluene in a 1 L flask equipped with a stirbar. To the stirred solution was carefully added 26.1 g (184 mmol)iodomethane and then was allowed to reflux for 2 hours. Next, thecondenser was removed and a stream of N₂ was passed over the flask toremove excess iodomethane. After the volume of the solution was reducedby half, it was cooled and the white solids collected on a filter andrinsed with toluene followed by hexanes. 35.0 g1-(N-[2-methylbenzimidazole])-4-(o-tolyl)benzene was achieved afterdrying in vacuo (90.0% yield).

Step 3: Synthesis of fac(mer)-iridium(III)tris(1-(N-[2-methylbenzimidazole])-4-(o-tolyl)benzene

250 mL 2-methoxyethanol was purged with N₂ and refluxed in a 500 mLround bottom flask equipped with a stir bar. After deoxygenation of thesolvent, the flask was cooled and 10.0 g (23.5 mmol)1-(N-[2-methylbenzimidazole])-4-(o-tolyl)benzene was added along with2.1 g (5.86 mmol) IrCl₃.3H₂O and 4.1 g (17.6 mmol) silver(I) oxide. Themixture was refluxed for 30 minutes whereupon an additional 4.1 g (17.6mmol) silver(I) oxide was added. After heating for an additional 1.5hours at reflux, HPLC indicated favorable formation of both the mer andfac isomers (˜1:2 ratio). The reaction mixture was cooled and filtered.Whereas the filtrate was put to the side (mer-rich), the resultingsolids were removed the filter, stirred vigorously in 300 mL MeCl₂ andfiltered again. This process was performed a second time and the 600 mLof dichloromethane filtrates were evaporated of solvent to give beigesolids that were purified on a silica gel column using a gradient of 20%dichloromethane→100% dichloromethane to give ˜2.5 g iridium(III)tris(1-(N-[2-methylbenzimidazole])-4-(o-tolyl)benzene as off-whitesolids. This material was sublimed in vacuo and afforded 1.5 g to betested in an OLED device (97.8% HPLC assay, ¹HNMR confirmed, λ_(max)emission: 466 nm, ox: 0.52 (r), 77K excited state lifetime: 67 μsec).The mer-rich filtrate was evaporated of solvent and the residuesolubilized in dichloromethane and purified on a column of silica usinga gradient of EtoAc/hexanes as eluent. The mer fractions were evaporatedof solvent to give ˜1.0 g mer iridium(III)tris(1-(N-[2-methylbenzimidazole])-4-(o-tolyl)benzene as an off-whitesolid (98.0% mer) that converted to fac (3:1 mer:fac) upon sublimation(¹H NMR confirmed, λ_(max) emission: 468 nm, ox: 0.43 (i), 77K excitedstate lifetime: 48 μsec).

Device Fabrication and Measurement

All devices were fabricated by high vacuum (<10-7 Torr) thermalevaporation. The anode electrode was ˜1200 Å of indium tin oxide (ITO).The cathode consisted of 10 Å of LiF followed by 1,000 Å of Al. Alldevices were encapsulated with a glass lid sealed with an epoxy resin ina nitrogen glove box (<1 ppm of H20 and O2) immediately afterfabrication, and a moisture getter was incorporated inside the package.

The following table indicates the dopants used for the devices ofExamples 6-21.

Dopant A fac-

Dopant B fac-

Dopant C fac-

Dopant D mer-

Dopant E fac-

Dopant F mer-

Dopant G fac-

Dopant H fac-

Dopant I fac-

Dopant J mer-

Dopant K fac-

Dopant L mer-

Dopant M fac-

Dopant N mer-

Dopant O fac-

Dopant P mer-

EXAMPLE 6

The organic stack consisted of sequentially, from the ITO surface, 100 Åthick of copper phthalocyanine (CuPc) as the hole injection layer (HIL),300 Å of 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (α-NPD), as thehole transporting layer (HTL), 300 Å of UGH5 doped with 6 wt % Dopant Aas the emissive layer (EML), 400 Å ofaluminum(III)bis(2-methyl-8-hydroxyquinolinato)4-phenylphenolate (BAlq)as the electron transporting layer (ETL).

EXAMPLE 7

The organic stack consisted of sequentially, from the ITO surface, 100 Åthick of copper phthalocyanine (CuPc) as the hole injection layer (HIL),300 Å of 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (α-NPD), as thehole transporting layer (HTL), 300 Å of UGH5 doped with 6 wt % Dopant Bas the emissive layer (EML), 400 Å ofaluminum(III)bis(2-methyl-8-hydroxyquinolinato)4-phenylphenolate (BAlq)as the electron transporting layer (ETL).

FIG. 3 shows plots of current vs. voltage for deviceCuPc(100)/NPD(300)/UGH5: Dopant A (6%, 300)/BAlQ(400) and deviceCuPc(100)/NPD(300)/UGH5: Dopant B (6%, 300)/BAlQ(400).

FIG. 4 shows plots of quantum efficiency vs. current density for deviceCuPc(100)/NPD(300)/UGH5: Dopant A (6%, 300)/BAlQ(400) and deviceCuPc(100)/NPD(300)/UGH5: Dopant B (6%, 300)/BAlQ(400).

FIG. 5 shows plots of the electroluminescent spectra of deviceCuPc(100)/NPD(300)/UGH5: Dopant A (6%, 300)/BAlQ(400) and deviceCuPc(100)/NPD(300)/UGH5: Dopant B (6%, 300)/BAlQ(400).

EXAMPLE 8

The organic stack consisted of sequentially, from the ITO surface, 100 Åthick of copper phthalocyanine (CuPc) as the hole injection layer (HIL),300 Å of 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (α-NPD), as thehole transporting layer (HTL), 300 Å of mCBP doped with 6 wt % Dopant Cas the emissive layer (EML), 400 Å ofaluminum(III)bis(2-methyl-8-hydroxyquinolinato)4-phenylphenolate (BAlq)as the electron transporting layer (ETL).

EXAMPLE 9

The organic stack consisted of sequentially, from the ITO surface, 100 Åthick of copper phthalocyanine (CuPc) as the hole injection layer (HIL),300 Å of 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (α-NPD), as thehole transporting layer (HTL), 300 Å of mCBP doped with 6 wt % Dopant Das the emissive layer (EML), 400 Å ofaluminum(III)bis(2-methyl-8-hydroxyquinolinato)4-phenylphenolate (BAlq)as the electron transporting layer (ETL).

FIG. 6 shows plots of current vs. voltage for deviceCuPc(100)/NPD(300)/mCBP: Dopant C (6%, 300)/BAlQ(400) and deviceCuPc(100)/NPD(300)/mCBP:Dopant D (6%, 300)/BAlQ(400).

FIG. 7 shows plots of quantum efficiency vs. current density for deviceCuPc(100)/NPD(300)/mCBP: Dopant C (6%, 300)/BAlQ(400) and deviceCuPc(100)/NPD(300)/mCBP: Dopant D (6%, 300)/BAlQ(400).

FIG. 8 shows plots of the electroluminescent spectra of deviceCuPc(100)/NPD(300)/mCBP: Dopant C (6%, 300)/BAlQ(400) and deviceCuPc(100)/NPD(300)/mCBP: Dopant D (6%, 300)/BAlQ(400).

FIG. 24 shows plots of the plot of operation lifetime of deviceCuPc(100)/NPD(300)/UGH5: Dopant D (6%, 300)/BAlQ(400).

EXAMPLE 10

The organic stack consisted of sequentially, from the ITO surface, 100 Åthick of copper phthalocyanine (CuPc) as the hole injection layer (HIL),300 Å of 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (α-NPD), as thehole transporting layer (HTL), 300 Å of mCBP doped with 6 wt % Dopant Eas the emissive layer (EML), 400 Å ofaluminum(III)bis(2-methyl-8-hydroxyquinolinato)4-phenylphenolate (BAlq)as the electron transporting layer (ETL).

EXAMPLE 11

The organic stack consisted of sequentially, from the ITO surface, 100 Åthick of copper phthalocyanine (CuPc) as the hole injection layer (HIL),300 Å of 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (α-NPD), as thehole transporting layer (HTL), 300 Å of mCBP doped with 6 wt % Dopant Fas the emissive layer (EML), 400 Å ofaluminum(III)bis(2-methyl-8-hydroxyquinolinato)4-phenylphenolate (BAlq)as the electron transporting layer (ETL).

FIG. 9 shows plots of current vs. voltage for deviceCuPc(100)/NPD(300)/mCBP: Dopant E (300, 6%)/BAlQ(400) and deviceCuPc(100)/NPD(300)/mCBP: Dopant F (300, 6%)/BAlQ(400).

FIG. 10 shows plots of quantum efficiency vs. current density for deviceCuPc(100)/NPD(300)/mCBP: Dopant E (300, 6%)/BAlQ(400) and deviceCuPc(100)/NPD(300)/mCBP: Dopant F (300, 6%)/BAlQ(400).

FIG. 11 shows plots of the electroluminescent spectra of deviceCuPc(100)/NPD(300)/mCBP: Dopant E (300, 6%)/BAlQ(400) and deviceCuPc(100)/NPD(300)/mCBP: Dopant F (300, 6%)/BAlQ(400).

EXAMPLE 12

The organic stack consisted of sequentially, from the ITO surface, 100 Åthick of copper phthalocyanine (CuPc) as the hole injection layer (HIL),300 Å of 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (α-NPD), as thehole transporting layer (HTL), 300 Å of UGH5 doped with 6 wt % Dopant Gas the emissive layer (EML), 400 Å ofaluminum(III)bis(2-methyl-8-hydroxyquinolinato)4-phenylphenolate (BAlq)as the electron transporting layer (ETL).

FIG. 12 shows plots of current vs. voltage for deviceCuPc(100)/NPD(300)/UGH5: Dopant G (6%, 300)/BAlQ(400).

FIG. 13 shows plots of quantum efficiency vs. current density forCuPc(100)/NPD(300)/UGH5: Dopant G (6%, 300)/BAlQ(400).

FIG. 14 shows plots of the electroluminescent spectra ofCuPc(100)/NPD(300)/UGH5: Dopant G (6%, 300)/BAlQ(400).

EXAMPLE 13

The organic stack consisted of sequentially, from the ITO surface, 100 Åthick of copper phthalocyanine (CuPc) as the hole injection layer (HIL),300 Å of 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (α-NPD), as thehole transporting layer (HTL), 300 Å of mCP doped with 6 wt % Dopant Has the emissive layer (EML), 400 Å ofaluminum(III)bis(2-methyl-8-hydroxyquinolinato)4-phenylphenolate (BAlq)as the electron transporting layer (ETL).

EXAMPLE 14

The organic stack consisted of sequentially, from the ITO surface, 100 Åthick of copper phthalocyanine (CuPc) as the hole injection layer (HIL),300 Å of 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (α-NPD), as thehole transporting layer (HTL), 300 Å of CBP doped with 6 wt % Dopant Ias the emissive layer (EML), 400 Å ofaluminum(III)bis(2-methyl-8-hydroxyquinolinato)4-phenylphenolate (BAlq)as the electron transporting layer (ETL).

EXAMPLE 15

The organic stack consisted of sequentially, from the ITO surface, 100 Åthick of copper phthalocyanine (CuPc) as the hole injection layer (HIL),300 Å of 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (α-NPD), as thehole transporting layer (HTL), 300 Å of mCBP doped with 6 wt % Dopant Jas the emissive layer (EML), 400 Å ofaluminum(III)bis(2-methyl-8-hydroxyquinolinato)4-phenylphenolate (BAlq)as the electron transporting layer (ETL).

FIG. 15 shows plots of current vs. voltage for deviceCuPc(100)INPD(300)/mCP: Dopant H (6%, 300)/BAIQ(400), deviceCuPc(100)/NPD(300)/CBP: Dopant I (6%, 300)/BAIQ(400), and deviceCuPc(100)/NPD(300)/mCBP: Dopant J(6%, 300)/BAlQ(400).

FIG. 16 shows plots of quantum efficiency vs. current density for deviceCuPc(100)/NPD(300)/mCP: Dopant H (6%, 300)/BAlQ(400), deviceCuPc(100)/NPD(300)/CBP: Dopant I (6%, 300)/BAlQ(400), and deviceCuPc(100)/NPD(300)/mCBP: Dopant J (6%, 300)/BAlQ(400).

FIG. 17 shows plots of the electroluminescent spectra of deviceCuPc(100)/NPD(300)/mCP: Dopant H (6%, 300)/BAlQ(400), deviceCuPc(100)/NPD(300)/CBP: Dopant I (6%, 300)/BAlQ(400), and deviceCuPc(100)/NPD(300)/mCBP: Dopant J (6%, 300)/BAlQ(400).

EXAMPLE 16

The organic stack consisted of sequentially, from the ITO surface, 100 Åthick of copper phthalocyanine (CuPc) as the hole injection layer (HIL),300 Å of 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (α-NPD), as thehole transporting layer (HTL), 300 Å of mCBP doped with 6 wt % Dopant Kas the emissive layer (EML), 400 Å ofaluminum(III)bis(2-methyl-8-hydroxyquinolinato)4-phenylphenolate (BAlq)as the electron transporting layer (ETL).

EXAMPLE 17

The organic stack consisted of sequentially, from the ITO surface, 100 Åthick of copper phthalocyanine (CuPc) as the hole injection layer (HIL),300 Å of 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (α-NPD), as thehole transporting layer (HTL), 300 Å of mCBP doped with 6 wt % Dopant Las the emissive layer (EML), 400 Å ofaluminum(III)bis(2-methyl-8-hydroxyquinolinato)4-phenylphenolate (BAlq)as the electron transporting layer (ETL).

EXAMPLE 18

The organic stack consisted of sequentially, from the ITO surface, 100 Åthick of copper phthalocyanine (CuPc) as the hole injection layer (HIL),300 Å of 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (α-NPD), as thehole transporting layer (HTL), 300 Å of UGH5 doped with 6 wt % Dopant Mas the emissive layer (EML), 400 Å ofaluminum(III)bis(2-methyl-8-hydroxyquinolinato)4-phenylphenolate (BAlq)as the electron transporting layer (ETL).

FIG. 18 shows plots of current vs. voltage for deviceCuPc(100)/NPD(300)/mCBP: Dopant K (6%, 300)/BAlQ(400), deviceCuPc(100)/NPD(300)/mCBP: Dopant L (6%, 300)/BAlQ(400), and deviceCuPc(100)/NPD(300)/UGH5: Dopant M (6%, 300)/BAlQ(400).

FIG. 19 shows plots of quantum efficiency vs. current density for deviceCuPc(100)/NPD(300)/mCBP: Dopant K (6%, 300)/BAlQ(400), deviceCuPc(100)/NPD(300)/mCBP: Dopant L (6%, 300)/BAlQ(400), and deviceCuPc(100)/NPD(300)/UGH5: Dopant M (6%, 300)/BAlQ(400).

FIG. 20 shows plots of the electroluminescent spectra of deviceCuPc(100)/NPD(300)/mCBP: Dopant K (6%, 300)/BAlQ(400), deviceCuPc(100)/NPD(300)/mCBP: Dopant L (6%, 300)/BAlQ(400), and deviceCuPc(100)/NPD(300)/UGH5: Dopant M (6%, 300)/BAlQ(400).

EXAMPLE 19

The organic stack consisted of sequentially, from the ITO surface, 100 Åthick of copper phthalocyanine (CuPc) as the hole injection layer (HIL),300 Å of 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (α-NPD), as thehole transporting layer (HTL), 300 Å of UGH5 doped with 6 wt % Dopant Nas the emissive layer (EML), 400 Å ofaluminum(III)bis(2-methyl-8-hydroxyquinolinato)4-phenylphenolate (BAlq)as the electron transporting layer (ETL).

EXAMPLE 20

The organic stack consisted of sequentially, from the ITO surface, 100 Åthick of copper phthalocyanine (CuPc) as the hole injection layer (HIL),300 Å of 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (α-NPD), as thehole transporting layer (HTL), 300 Å of UGH5 doped with 6 wt % Dopant Oas the emissive layer (EML), 400 Å ofaluminum(III)bis(2-methyl-8-hydroxyquinolinato)4-phenylphenolate (BAlq)as the electron transporting layer (ETL).

EXAMPLE 21

The organic stack consisted of sequentially, from the ITO surface, 100 Åthick of copper phthalocyanine (CuPc) as the hole injection layer (HIL),300 Å of 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (α-NPD), as thehole transporting layer (HTL), 300 Å of UGH5 doped with 6 wt % Dopant Pas the emissive layer (EML), 400 Å ofaluminum(III)bis(2-methyl-8-hydroxyquinolinato)4-phenylphenolate (BAlq)as the electron transporting layer (ETL).

FIG. 21 shows plots of current vs. voltage for deviceCuPc(100)/NPD(300)/UGH5: Dopant N (6%, 300)/BAlQ(400), deviceCuPc(100)/NPD(300)/UGH5: Dopant O (6%, 300)/BAlQ(400), and deviceCuPc(100)/NPD(300)/UGH5: Dopant P (6%, 300)/BAlQ(400).

FIG. 22 shows plots of quantum efficiency vs. current density for deviceCuPc(100)/NPD(300)/UGH5: Dopant N (6%, 300)/BAlQ(400), deviceCuPc(100)/NPD(300)/UGH5: Dopant O (6%, 300)/BAlQ(400), and deviceCuPc(100)/NPD(300)/UGH5: Dopant P (6%, 300)/BAlQ(400).

FIG. 23 shows plots of the electroluminescent spectra of deviceCuPc(100)/NPD(300)/UGH5: Dopant N (6%, 300)/BAlQ(400), deviceCuPc(100)/NPD(300)/UGH5: Dopant O (6%, 300)/BAlQ(400), and deviceCuPc(100)/NPD(300)/UGH5: Dopant P (6%, 300)/BAlQ(400).

Synthesis of Carbene Dopants

EXAMPLE 22, 23 Synthesis of fac-iridium(III) tris(1-(2-naphthyl-3-methyl-benzimidazoline-2-ylidene-C,C²) andmer-iridium(III) tris(1-(2-naphthyl-3-methyl-benzimidazoline-2-ylidene-C,C²) [Dopant C, D]

A 3 L round-bottomed flask was charged with 113.83 g of silver(I) oxide,38 g of 1-(2-naphthyl)-3-methyl-benzimidazolate iodide, 9.1 g of iridiumtrichloride hydrate, and 2000 mL of 2-ethoxyethanol. The reaction wasstirred and heated at 120oC for 2 h under nitrogen while protected fromlight with aluminum foil. The reaction mixture was cooled to ambienttemperature and concentrated under reduced pressure (20 mmHg).Filtration through Celite using dichloromethane as the eluent wasperformed to remove the silver(I) salts. A light brown solution wasobtained and further purified by flash column chromatography on silicagel using dichloromethane as the eluent and 6 g (25.2%) of iridiumcomplex was obtained. The mer isomer was selectively crystallized from amixture of dichloromethane and methanol. The mother liquid wasevaporated to dryness and the residue was recrystallized fromdichloromethane to obtain the fac isomer.

EXAMPLE 24 Synthesis of fac-iridium(III) tris(1-(4,5-dimethylphenyl)-3-methyl-benzimidazoline-2-ylidene-C,C²) [DopantA]

A 3 L round-bottomed flask was charged with 66.74 g of silver(I) oxide,21 g of 1-(4,5-dimethylphenyl)-3-methyl-benzimidazolate iodide, 5.33 gof iridium trichloride hydrate, molecular sieve (200 g) and 1108 mL of2-ethoxyethanol. The reaction was stirred and heated at 120° C. for 3.5h under nitrogen while protected from light with aluminum foil. Thereaction mixture was cooled to ambient temperature and concentratedunder reduced pressure (20 mmHg). Filtration through Celite usingdichloromethane as the eluent was performed to remove the silver(I)salts. A light brown solution was obtained and further purified by flashcolumn chromatography on silica gel using dichlormethane as the eluentand 9 g (60%) of iridium complex was obtained. The fac isomer wasselectively crystallized from a mixture of dichloromethane and methanol.

EXAMPLE 25 Synthesis of fac-iridium(III) tris(1-(4,5-dimethylphenyl)-3-methyl-5,6-dimethyl-benzimidazoline-2-ylidene-C,C²)[Dopant B]

A 3 L round-bottomed flask was charged with 118.14 g of silver(I) oxide,40 g of 1-(4,5-dimethylphenyl)-3-methyl-benzimidazolate iodide, 9.44 gof iridium trichloride hydrate, molecular sieve (100 g) and 2000 mL of2-ethoxyethanol. The reaction was stirred and heated at 120° C. for 3 hunder nitrogen while protected from light with aluminum foil. Thereaction mixture was cooled to ambient temperature and concentratedunder reduced pressure (20 mmHg). Filtration through Celite usingdichloromethane as the eluent was performed to remove the silver(I)salts. A light brown solution was obtained and further purified by flashcolumn chromatography on silica gel using dichloromethane as the eluentand 9 g (35%) of iridium complex was obtained. The fac isomer wasselectively crystallized from a mixture of dichloromethane and methanol.

EXAMPLE 26 Synthesis of fac-iridium(III) tris(1-(4-chlorolphenyl)-3-(4-chlorophenyl)-imidazoline-2-ylidene-C,C²)

A 100 ml round-bottomed flask was charged with 177.9 mg of silver(I)oxide, 250 mg of 1,3-bis(4-chlorophenyl)imidazolium Chloride, 71.09 mgof iridium trichloride hydrate, and 20 mL of 2-ethoxyethanol. Thereaction was stirred and heated at 120° C. for 1 h under nitrogen whileprotected from light with aluminum foil. The reaction mixture was cooledto ambient temperature and concentrated under reduced pressure (20mmHg). Filtration through Celite using dichloromethane as the eluent wasperformed to remove the silver(I) salts. A light brown solution wasobtained and further purified by flash column chromatography on silicagel using dichloromethane as the eluent and 50 mg (25%) of iridiumcomplex was obtained. The fac isomer was selectively crystallized from amixture of dichloromethane and methanol.

While the present invention is described with respect to particularexamples and preferred embodiments, it is understood that the presentinvention is not limited to these examples and embodiments. The presentinvention as claimed therefore includes variations from the particularexamples and preferred embodiments described herein, as will be apparentto one of skill in the art.

1. An organic light emitting device, comprising: (a) an anode; (b) acathode; (c) an organic layer disposed between the anode and thecathode, wherein the organic layer comprises a compound comprising oneor more carbene ligands coordinated to a metal center, wherein thecompound has the structure:

wherein M is a metal; R₁ is independently selected from hydrogen, alkyl,alkenyl, alkynyl, aralkyl, aryl, heteroaryl, substituted aryl,substituted heteroaryl, or a heterocyclic group; R₂, R₃, R₈, R₁₀, R₁₁,and R₁₂ are independently selected from hydrogen, alkyl, alkenyl,alkynyl, aralkyl, CN, CF₃, CO₂R′, C(O)R′, C(O)NR′₂, NR′₂, NO₂, OR′, SR′,SO₂, SOR′, SO₃R′, halo, aryl, heteroaryl, substituted aryl, substitutedheteroaryl, or a heterocyclic group; and additionally or alternatively,one or more of R₁ and R₂, and R₂ and R₃, and two R₈ groups, two R₁₁groups, and two R₁₂ groups on the same ring together form independentlya 5 or 6-member cyclic group, wherein said cyclic group is cycloalkyl,cycloheteroalkyl, aryl or heteroaryl; and wherein said cyclic group isoptionally substituted by one or more substituents J; each substituent Jis independently selected from the group consisting of R′, CN, CF₃,C(O)OR′, C(O)R′, C(O)NR′₂, NR′₂, NO₂, OR′, SR′, SO₂, SOR′, or SO₃R′, andadditionally, or alternatively, two J groups on adjacent ring atoms forma fused 5- or 6-membered aromatic group; each R′ is independentlyselected from halo, H, alkyl, alkenyl, alkynyl, heteroalkyl, aralkyl,aryl and heteroaryl; (X-Y) is selected from a photoactive ligand or anancillary ligand; d is 0, 1, 2, 3, or 4; m is a value from 1 to themaximum number of ligands that may be attached to metal M; m+n is themaximum number of ligands that may be attached to metal M.
 2. The deviceof claim 1, wherein the compound is selected from the group consistingof


3. The device of claim 2, wherein M is selected from the groupconsisting of Ir, Pt, Pd, Rh, Re, Ru, Os, Tl, Pb, Bi, In, Sn, Sb, Te,Au, and Ag.
 4. The device of claim 3, wherein M is Ir.
 5. The device ofclaim 4, wherein m is 3 and n is
 0. 6. An organic light emitting device,comprising: (a) an anode; (b) a cathode; (c) an organic layer disposedbetween the anode and the cathode, wherein the organic layer comprises acompound comprising a carbene ligand having the structure:

wherein R₁ is independently selected from hydrogen, alkyl, alkenyl,alkynyl, aralkyl, aryl, heteroaryl, substituted aryl, substitutedheteroaryl, or a heterocyclic group; R₂, R₃, R₈, R₁₀, R₁₁ and R₁₂ areindependently selected from hydrogen, alkyl, alkenyl, alkynyl, aralkyl,CN, CF₃, CO₂R′, C(O)R′, C(O)NR′₂, NR′₂, NO₂, OR′, SR′, SO₂, SOR′, SO₃R′,halo, aryl, heteroaryl, substituted aryl, substituted heteroaryl, or aheterocyclic group; and additionally or alternatively, one or more of R₁and R₂, and R₃, and two R₈ groups, two R₁₁ groups, and two R₁₂ groups onthe same ring together form independently a 5 or 6-member cyclic group,wherein said cyclic group is cycloalkyl, cycloheteroalkyl, aryl orheteroaryl; and wherein said cyclic group is optionally substituted byone or more substituents J; each substituent J is independently selectedfrom the group consisting of R′, CN, CF₃, C(O)OR′, C(O)R′, C(O)NR′₂,.NR₂, NO₂, OR′, SR′, SO₂, SOR′, or SO₃R′, and additionally, oralternatively, two J groups on adjacent ring atoms form a fused 5- or6-membered aromatic group; each R′ is independently selected from halo,H, alkyl, alkenyl, alkynyl, heteroalkyl, aralkyl, aryl and heteroaryl;wherein the ligand is coordinated to a metal; and d is 0, 1, 2, 3, or 4.7. The device of claim 6, wherein the ligand is selected from the groupconsisting of:


8. A compound selected from the group consisting of:

wherein R₁ is independently selected from hydrogen, alkyl, alkenyl,alkynyl, aralkyl, aryl, heteroaryl, substituted aryl, substitutedheteroaryl, or a heterocyclic group; R₂, R₃, R₈, R₁₀, R₁₁, and R₁₂ areindependently selected from hydrogen, alkyl, alkenyl, alkynyl, aralkyl,CN, CF₃, CO₂R′, C(O)R′C(O)NR′₂, NR′₂, NO₂, OR′, SR′, SO₂, SOR′, SO₃R′,halo, aryl, heteroaryl, substituted aryl, substituted heteroaryl, or aheterocyclic group; and additionally or alternatively, one or more of R₁and R₂, and R₂ and R₃, and two R₈ groups, two R₁₁ groups, and two R₁₂groups on the same ring together form independently a 5 or 6-membercyclic group, wherein said cyclic group is cycloalkyl, cycloheteroalkyl,aryl or heteroaryl; and wherein said cyclic group is optionallysubstituted by one or more substituents J; each substituent J isindependently selected from the group consisting of R′, CN, CF₃,C(O)OR′, C(O)R′, C(O)NR′, NR′₂, NO₂, OR′, SR′, SO₂, SOR′, or SO₃R′, andadditionally, or alternatively, two J groups on adjacent ring atoms forma fused 5 -or 6-membered aromatic group; each R′ is independentlyselected from halo, H, alkyl, alkenyl, alkynyl, heteroalkyl, aralkyl,aryl and heteroaryl; (X-Y) is selected from a photoactive ligand or anancillary ligand, d is 0, 1, 2, 3, or 4; m is a value from 1 to themaximum number of ligands that may be attached to metal M; m+n is themaximum number of ligands that may be attached to metal M.
 9. Thecompound of claim 8, selected from the group consisting of:


10. The compound of claim 9, wherein M is selected from the groupconsisting of Ir, Pt, Pd, Rh, Re, Ru, Os, Tl, Pb, Bi, In, Sn, Sb, Te,Au, and Ag.
 11. The compound of claim 10, wherein M is Ir.
 12. Thecompound of claim 11, wherein m is 3 and n is 0.